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1 <html>
2 <head>
3 <title>pcre specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcre man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18 <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 </ul>
23 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24 <P>
25 The PCRE library is a set of functions that implement regular expression
26 pattern matching using the same syntax and semantics as Perl, with just a few
27 differences. Some features that appeared in Python and PCRE before they
28 appeared in Perl are also available using the Python syntax, there is some
29 support for one or two .NET and Oniguruma syntax items, and there is an option
30 for requesting some minor changes that give better JavaScript compatibility.
31 </P>
32 <P>
33 The current implementation of PCRE corresponds approximately with Perl
34 5.10/5.11, including support for UTF-8 encoded strings and Unicode general
35 category properties. However, UTF-8 and Unicode support has to be explicitly
36 enabled; it is not the default. The Unicode tables correspond to Unicode
37 release 5.2.0.
38 </P>
39 <P>
40 In addition to the Perl-compatible matching function, PCRE contains an
41 alternative function that matches the same compiled patterns in a different
42 way. In certain circumstances, the alternative function has some advantages.
43 For a discussion of the two matching algorithms, see the
44 <a href="pcrematching.html"><b>pcrematching</b></a>
45 page.
46 </P>
47 <P>
48 PCRE is written in C and released as a C library. A number of people have
49 written wrappers and interfaces of various kinds. In particular, Google Inc.
50 have provided a comprehensive C++ wrapper. This is now included as part of the
51 PCRE distribution. The
52 <a href="pcrecpp.html"><b>pcrecpp</b></a>
53 page has details of this interface. Other people's contributions can be found
54 in the <i>Contrib</i> directory at the primary FTP site, which is:
55 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
56 </P>
57 <P>
58 Details of exactly which Perl regular expression features are and are not
59 supported by PCRE are given in separate documents. See the
60 <a href="pcrepattern.html"><b>pcrepattern</b></a>
61 and
62 <a href="pcrecompat.html"><b>pcrecompat</b></a>
63 pages. There is a syntax summary in the
64 <a href="pcresyntax.html"><b>pcresyntax</b></a>
65 page.
66 </P>
67 <P>
68 Some features of PCRE can be included, excluded, or changed when the library is
69 built. The
70 <a href="pcre_config.html"><b>pcre_config()</b></a>
71 function makes it possible for a client to discover which features are
72 available. The features themselves are described in the
73 <a href="pcrebuild.html"><b>pcrebuild</b></a>
74 page. Documentation about building PCRE for various operating systems can be
75 found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
76 distribution.
77 </P>
78 <P>
79 The library contains a number of undocumented internal functions and data
80 tables that are used by more than one of the exported external functions, but
81 which are not intended for use by external callers. Their names all begin with
82 "_pcre_", which hopefully will not provoke any name clashes. In some
83 environments, it is possible to control which external symbols are exported
84 when a shared library is built, and in these cases the undocumented symbols are
85 not exported.
86 </P>
87 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
88 <P>
89 The user documentation for PCRE comprises a number of different sections. In
90 the "man" format, each of these is a separate "man page". In the HTML format,
91 each is a separate page, linked from the index page. In the plain text format,
92 all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
93 of searching. The sections are as follows:
94 <pre>
95 pcre this document
96 pcre-config show PCRE installation configuration information
97 pcreapi details of PCRE's native C API
98 pcrebuild options for building PCRE
99 pcrecallout details of the callout feature
100 pcrecompat discussion of Perl compatibility
101 pcrecpp details of the C++ wrapper
102 pcredemo a demonstration C program that uses PCRE
103 pcregrep description of the <b>pcregrep</b> command
104 pcrematching discussion of the two matching algorithms
105 pcrepartial details of the partial matching facility
106 pcrepattern syntax and semantics of supported regular expressions
107 pcreperform discussion of performance issues
108 pcreposix the POSIX-compatible C API
109 pcreprecompile details of saving and re-using precompiled patterns
110 pcresample discussion of the pcredemo program
111 pcrestack discussion of stack usage
112 pcresyntax quick syntax reference
113 pcretest description of the <b>pcretest</b> testing command
114 </pre>
115 In addition, in the "man" and HTML formats, there is a short page for each
116 C library function, listing its arguments and results.
117 </P>
118 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
119 <P>
120 There are some size limitations in PCRE but it is hoped that they will never in
121 practice be relevant.
122 </P>
123 <P>
124 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
125 compiled with the default internal linkage size of 2. If you want to process
126 regular expressions that are truly enormous, you can compile PCRE with an
127 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
128 distribution and the
129 <a href="pcrebuild.html"><b>pcrebuild</b></a>
130 documentation for details). In these cases the limit is substantially larger.
131 However, the speed of execution is slower.
132 </P>
133 <P>
134 All values in repeating quantifiers must be less than 65536.
135 </P>
136 <P>
137 There is no limit to the number of parenthesized subpatterns, but there can be
138 no more than 65535 capturing subpatterns.
139 </P>
140 <P>
141 The maximum length of name for a named subpattern is 32 characters, and the
142 maximum number of named subpatterns is 10000.
143 </P>
144 <P>
145 The maximum length of a subject string is the largest positive number that an
146 integer variable can hold. However, when using the traditional matching
147 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
148 This means that the available stack space may limit the size of a subject
149 string that can be processed by certain patterns. For a discussion of stack
150 issues, see the
151 <a href="pcrestack.html"><b>pcrestack</b></a>
152 documentation.
153 <a name="utf8support"></a></P>
154 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
155 <P>
156 From release 3.3, PCRE has had some support for character strings encoded in
157 the UTF-8 format. For release 4.0 this was greatly extended to cover most
158 common requirements, and in release 5.0 additional support for Unicode general
159 category properties was added.
160 </P>
161 <P>
162 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
163 the code, and, in addition, you must call
164 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
165 with the PCRE_UTF8 option flag, or the pattern must start with the sequence
166 (*UTF8). When either of these is the case, both the pattern and any subject
167 strings that are matched against it are treated as UTF-8 strings instead of
168 strings of 1-byte characters.
169 </P>
170 <P>
171 If you compile PCRE with UTF-8 support, but do not use it at run time, the
172 library will be a bit bigger, but the additional run time overhead is limited
173 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
174 </P>
175 <P>
176 If PCRE is built with Unicode character property support (which implies UTF-8
177 support), the escape sequences \p{..}, \P{..}, and \X are supported.
178 The available properties that can be tested are limited to the general
179 category properties such as Lu for an upper case letter or Nd for a decimal
180 number, the Unicode script names such as Arabic or Han, and the derived
181 properties Any and L&. A full list is given in the
182 <a href="pcrepattern.html"><b>pcrepattern</b></a>
183 documentation. Only the short names for properties are supported. For example,
184 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
185 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186 compatibility with Perl 5.6. PCRE does not support this.
187 <a name="utf8strings"></a></P>
188 <br><b>
189 Validity of UTF-8 strings
190 </b><br>
191 <P>
192 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
193 are (by default) checked for validity on entry to the relevant functions. From
194 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
195 themselves derived from the Unicode specification. Earlier releases of PCRE
196 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
197 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
198 U+10FFFF, excluding U+D800 to U+DFFF.
199 </P>
200 <P>
201 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
202 Unicode Standard says this: "The Low Surrogate Area does not contain any
203 character assignments, consequently no character code charts or namelists are
204 provided for this area. Surrogates are reserved for use with UTF-16 and then
205 must be used in pairs." The code points that are encoded by UTF-16 pairs are
206 available as independent code points in the UTF-8 encoding. (In other words,
207 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
208 UTF-8.)
209 </P>
210 <P>
211 If an invalid UTF-8 string is passed to PCRE, an error return
212 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
213 your strings are valid, and therefore want to skip these checks in order to
214 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
215 at run time, PCRE assumes that the pattern or subject it is given
216 (respectively) contains only valid UTF-8 codes. In this case, it does not
217 diagnose an invalid UTF-8 string.
218 </P>
219 <P>
220 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
221 happens depends on why the string is invalid. If the string conforms to the
222 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
223 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
224 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
225 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
226 the result is undefined. Your program may crash.
227 </P>
228 <P>
229 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
230 encoded in a UTF-8-like manner as per the old RFC, you can set
231 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
232 situation, you will have to apply your own validity check.
233 </P>
234 <br><b>
235 General comments about UTF-8 mode
236 </b><br>
237 <P>
238 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
239 UTF-8 character if the value is greater than 127.
240 </P>
241 <P>
242 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
243 characters for values greater than \177.
244 </P>
245 <P>
246 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
247 bytes, for example: \x{100}{3}.
248 </P>
249 <P>
250 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
251 </P>
252 <P>
253 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
254 but its use can lead to some strange effects. This facility is not available in
255 the alternative matching function, <b>pcre_dfa_exec()</b>.
256 </P>
257 <P>
258 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
259 test characters of any code value, but, by default, the characters that PCRE
260 recognizes as digits, spaces, or word characters remain the same set as before,
261 all with values less than 256. This remains true even when PCRE is built to
262 include Unicode property support, because to do otherwise would slow down PCRE
263 in many common cases. Note in particular that this applies to \b and \B,
264 because they are defined in terms of \w and \W. If you really want to test
265 for a wider sense of, say, "digit", you can use explicit Unicode property tests
266 such as \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
267 the character escapes work is changed so that Unicode properties are used to
268 determine which characters match. There are more details in the section on
269 <a href="pcrepattern.html#genericchartypes">generic character types</a>
270 in the
271 <a href="pcrepattern.html"><b>pcrepattern</b></a>
272 documentation.
273 </P>
274 <P>
275 7. Similarly, characters that match the POSIX named character classes are all
276 low-valued characters, unless the PCRE_UCP option is set.
277 </P>
278 <P>
279 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
280 (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
281 whether or not PCRE_UCP is set.
282 </P>
283 <P>
284 9. Case-insensitive matching applies only to characters whose values are less
285 than 128, unless PCRE is built with Unicode property support. Even when Unicode
286 property support is available, PCRE still uses its own character tables when
287 checking the case of low-valued characters, so as not to degrade performance.
288 The Unicode property information is used only for characters with higher
289 values. Even when Unicode property support is available, PCRE supports
290 case-insensitive matching only when there is a one-to-one mapping between a
291 letter's cases. There are a small number of many-to-one mappings in Unicode;
292 these are not supported by PCRE.
293 </P>
294 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
295 <P>
296 Philip Hazel
297 <br>
298 University Computing Service
299 <br>
300 Cambridge CB2 3QH, England.
301 <br>
302 </P>
303 <P>
304 Putting an actual email address here seems to have been a spam magnet, so I've
305 taken it away. If you want to email me, use my two initials, followed by the
306 two digits 10, at the domain cam.ac.uk.
307 </P>
308 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
309 <P>
310 Last updated: 22 October 2010
311 <br>
312 Copyright &copy; 1997-2010 University of Cambridge.
313 <br>
314 <p>
315 Return to the <a href="index.html">PCRE index page</a>.
316 </p>


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